Radome builders often use impact-resistant laminates for providing ballistic protection to microwave antennae. Typically, laminates made of aramid fibers (Kevlar®) and polyethylene fibers (Spectra®, HDPE) are utilized. International application WO 03/031901 discloses a nano-denier fibrous woven sheet that could be used for ballistic impact-resistance radome design. The impact-resistant laminates or woven sheets combined with structural layers of honeycomb or solid foam cores can form basically an almost transparent radome suited to a specific band of frequencies. U.S. Pat. No. 5,182,155 discloses a composite radome structure based on alternate layers of Spectra® and dielectric honeycomb.
U.S. Pat. No. 4,570,166, discloses a radome structure made of a perforated metal wall, in which each of the holes is filled with a dielectric plug, providing improved ballistic protection. Electromagnetic waves propagate through the perforations in a thick metal plate if the apertures are large enough—such that the waveguide generated by the single hole is above its cutoff frequency.
Such a metal plate could be made of ballistic resistant steel, and the plugs could be made of a ballistic resistant ceramic material (e.g. silicon nitride) together conferring low microwave loss characteristics. A main drawback associated with this approach is the high density of the steel structure that leads to excessive weight.
Another approach known in the art consists of homogeneous ceramic radomes. Such radomes are commonly used in high-temperature applications such as in high-speed missiles. However, proper manufacturing of such radomes is rather costly. Impact resistant ceramic materials are usually very hard, leading to difficulties in the mechanical processing of the radome. Moreover, the tangent-loss of these ceramic materials is sensitive to the details of the sintering process, so the process parameters need to be carefully controlled over the whole volume of the radome.
It is a well known fact that a dense array of small ceramic units embedded in a suitable dielectric matrix can serve as an effective ballistic shield. U.S. Pat. No. 6,112,635 discloses a composite armor plate made of a single layer of tightly packed touching ceramic cylinders bound together by a solidified material. EP 1,363,101A1 discloses a ballistic armor comprising an array of non-touching ceramic units packed together by a non-ceramic material. However, both patents U.S. Pat. No. 6,112,635, and EP 1,363,101A1 do not relate to antenna radomes, and therefore are not applicable to microwaves or millimeter waves.